Phosphodiesterases (PDEs) are a class of intracellular enzymes involved in the hydrolysis of the nucleotides cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) to their respective nucleotide monophosphates. These cyclic nucleotides serve as secondary messengers in several cellular pathways, regulating an array of intracellular processes within neurons of the central nervous system including the activation of cAMP- and cGMP-dependent protein kinases that produce subsequent phosphorylation of proteins involved in regulation of synaptic transmission, synaptic plasticity, neuronal differentiation and survival.
So far, only a single gene for PDE2, PDE2A, has been identified; however, multiple alternatively spliced isoforms of PDE2A, which include PDE2A1, PDE2A2, and PDE2A3, have been reported. PDE2A was identified as a unique family based on primary amino acid sequence and distinct enzymatic activity. The human PDE2A3 sequence was isolated in 1997 (Rosman et al., Isolation and characterization of human cDNAs encoding a cGMP-stimulated 3′,5′-cyclic nucleotide phosphodiesterase, Gene, 191 (1):89-95, 1997).
Inhibition of PDE2A demonstrates enhanced cognitive function across multiple preclinical models of cognitive performance that reflect improvements in recognition memory, social interactions and working memory, which are all deficient in schizophrenia (Boess et al., Inhibition of phosphodiesterase 2 increases neuronal cGMP, synaptic plasticity and memory performance, Neuropharmacology, 47(7):1081-92, 2004). PDE2A inhibition also improved cognitive deficits that develop in aging and Alzheimer's disease (Domek-Lopacinska and Strosznajder, The effect of selective inhibition of cyclic GMP hydrolyzing phosphodiesterases 2 and 5 on learning and memory processes and nitric oxide synthetase activity in brain during aging, Brain Research, 1216:68-77, 2008). Bayer has published the biochemical and behavioral profile of BAY 60-7550, reporting a role in PDE2 inhibition in cognitive disorders (Brandon et al., Potential CNS Applications for Phosphodiesterase Enzyme Inhibitors, Annual Reports in Medicinal Chemistry 42: 4-5, 2007). This compound showed significant potency at other PDE isoforms and had high clearance and limited brain penetration.
PDE2 inhibitors have also been reported to show efficacy in preclinical models of anxiety and depression (Masood et al., Anxiolytic effects of phosphodiesterase-2 inhibitors associated with increased cGMP signaling, JPET 331(2):690-699, 2009; Masood et al., Reversal of Oxidative Stress-Induced Anxiety by Inhibition of Phosphodiesterase-2 in Mice, JPET 326(2):369-379, 2008; Reierson et al., Repeated antidepressant therapy increases cyclic GMP signaling in rat hippocampus, Neurosci. Lett., 466(3):149-53, 2009).
PDE2A protein expressed in the dorsal horn of the spinal cord and dorsal root ganglia enables PDE2A to modulate cyclic nucleotide levels in these regions during processing of neuropathic and inflammatory pain (Schmidtko et al., cGMP Produced by NO-Sensitive Guanylyl Cyclase Essentially Contributes to Inflammatory and Neuropathic Pain by Using Targets Different from cGMP-Dependent Protein Kinase I, The Journal of Neuroscience, 28(34):8568-8576, 2008).
In the periphery, the expression of PDE2A in endothelial cells has been reported to play a critical role in regulation of endothelial barrier function. The expression levels of PDE2A in endothelial cells are increased in response to inflammatory cytokines such as TNF-alpha under conditions of sepsis and acute respiratory distress syndrome, and contribute to disruption of endothelial barrier function. Inhibition of PDE2A has been demonstrated to reverse permeability deficits in sepsis and enhance survival rates in animal models of sepsis and endotoxicosis (Seybold et al., Tumor necrosis factor-{alpha}-dependent expression of phosphodiesterase 2: role in endothelial hyperpermeability, Blood, 105:3569-3576, 2005; Kayhan et al., The adenosine deaminase inhibitor erythro-9-[2-hydroxyl-3-nonyl]-adenine decreases intestinal permeability and protects against experimental sepsis: a prospective, randomized laboratory investigation, Critical Care, 12 (5):R125, 2008).
At least for the reasons discussed hereinabove, new or improved agents that modulate (such as inhibiting/antagonizing) PDE2 are continually needed for developing new and more effective pharmaceuticals to treat PDE2-associated conditions or diseases or disorders, such as cognitive impairment associated with schizophrenia anxiety, depression, neuropathic pain, inflammatory pain, sepsis, and endotoxicosis, to name a few. In discovering new or improved agents that modulate (such as inhibiting/antagonizing) PDE2, it is also desirable but not required to discover agents with improved chemical or biological properties such as solubility, bioavailability, pharmacokinetics, pharmacodynamics, and/or toxicity. It is also desirable but not required to discover agents with less side effects such as less cardiovascular side effects. In some embodiments, the compounds, compositions, and methods described herein are directed, in part, toward these unmet needs.
Cytochrome P450 enzymes are among the most important enzymes in the metabolism of drugs and other xenobiotics. They comprise a large family of enzymes, of which the most important are the 1A, 2A, 2B, 2C, 2D, 2E, and 3A groups. The drug metabolism reactions catalyzed by these enzymes are mostly oxidations, including hydroxylation, heteroatom dealkylation, and dehydrogenation reactions, among others. These reactions serve to make drugs more hydrophilic (and, hence, more readily excreted) and to add functional groups to molecules that can be subsequently coupled with hydrophilic endogenous entities (such as glucuronic acid or sulfuric acid groups) and excreted. P450 enzymes are a source of interpatient variability of drug pharmacokinetics, due to drug-drug interactions or naturally occurring genetic variants in the structure and/or expression of these enzymes.
Among the many human P450 enzyme families, the CYP3A family is most important, since it is involved in the metabolism of hundreds of compounds. CYP3A has four members in humans: CYP3A4, CYP3A5, CYP3A7, and CYP3A43. CYP3A4 and CYP3A5 are important in the metabolism of drugs. These two enzymes are similar in amino acid sequence, but some differences in substrate profile exist. CYP3A5 is subject to a genetic polymorphism, with some subjects not expressing a functional protein. This can result in interindividual differences in therapy and toxicity.
Because of the importance of CYP3A4 and CYP3A5, it is desirable to develop tools and methods that can be used to distinguish the contribution of each of these enzymes in the metabolism of drugs. Due to their high structural similarity, a tool and method to determine the relative contribution of 3A4 versus 3A5 to the metabolism of a new chemical has not been forthcoming. [See e.g. Obach, et. al, “Mechanism-Based Inactivation of Human Cytochrome P450 Enzymes and the Prediction of Drug-Drug Interactions,” Drug Metabolism And Disposition, Vol. 35, No 2, pp 246-255, 2007]. In some embodiments, compounds, compositions, and methods described herein are directed, in part, toward these unmet needs.